dm: pci: Add APIs to find next capability and extended capability

This introduces two new APIs dm_pci_find_next_capability() and
dm_pci_find_next_ext_capability() to get PCI capability address
and PCI express extended capability address for a given PCI device
starting from a given offset.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>